There is no curative therapy for more than 90% of patients with myeloid leukemias. We have developed a mouse monoclonal antibody M195 (mAb M195), anti-CD33 (mAb M195) that is specifically reactive with acute myelogenous leukemia (AML) cells and myeloid progenitor cells, but not with normal tissues or hematopoietic stem cells. Our pilot trials in humans have shown that mAb M195 can rapidly, efficiently, and quantitatively target to AML cells in the blood and bone marrow, and that a large portion of the radiolabel tracer tagged to the IgG can be delivered into the target cells. Mouse M195 is not cytotoxic, but when small doses of (131)I(40 mCi on 4 mg) were attached, hundreds of billions of leukemia cells were killed without side effects. The goal of this proposal is to develop potent recombinant, cytotoxic constructs of M195 using recently developed molecular genetic techniques and new radiochemistry. Three types of recombinant molecules based on the mouse M195 will be characterized and compared to the original M195 IgG: (1) Genetically engineered single chain M195. (2) Human-mouse chimeric IgG with human- derived constant regions for IgG1 and IgG3 and (3) fully humanized M195 retaining only the hypervariable regions and other sterically important amino-acids from the mouse Ig. Each construct will be assessed for immunoreactivity, specificity, avidity, stability and pharmacology in vitro and in model systems using cell lines, fresh leukemia samples and bone marrows. The unique specificity will allow us to assay killing of fresh leukemia cells or cell lines versus toxicity to normal progenitors when mixed into normal marrow cultures. The constructs will be radiolabeled with (131)I for potential applications for bone marrow ablation as well as with (123)I and chelated (212)Bi and (212)Pb, which have potential applications for single cell killing. Internalization kinetics and microdosimetry will be studied in vitro. using cell fractionation. Efforts will be made to optimize killing by use of drugs to enhance entry and retention of isotopes in the cytoplasm and by use of agents such as interferons, growth factors and other factors to increase cell surface expression of CD33.